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          <h1 class="post-title" itemprop="name headline">源码分析 RocketMQ DLedger 多副本存储实现</h1>
        

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        <div id="vip-container"><p>RocketMQ DLedger 的存储实现思路与 RocketMQ 的存储实现思路相似，本文就不再从源码角度详细剖析其实现，只是点出其实现关键点。我们不妨简单回顾一下 CommitLog 文件、ConsumeQueue 文件设计思想。</p>
<p>其文件组成形式如下：<br><img src="https://img-blog.csdnimg.cn/20190831215052708.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70" alt="在这里插入图片描述"><br>正如上图所示，多个 commitlog 文件组成一个逻辑上的连续文件，使用 MappedFileQueue 表示，单个 commitlog 文件使用 MappedFile 表示。</p>
<blockquote>
<p>温馨提示：如果想详细了解 RocketMQ 关于存储部分的讲解，可以关注笔者的《RocketMQ 技术内幕》一书。</p>
</blockquote>
<h2 id="1、DLedger-存储相关类图"><a href="#1、DLedger-存储相关类图" class="headerlink" title="1、DLedger 存储相关类图"></a>1、DLedger 存储相关类图</h2><p><img src="https://img-blog.csdnimg.cn/20190831215151796.jpg?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70" alt="在这里插入图片描述"></p>
<h3 id="1-1-DLedgerStore"><a href="#1-1-DLedgerStore" class="headerlink" title="1.1 DLedgerStore"></a>1.1 DLedgerStore</h3><p>存储抽象类，定义如下核心方法：</p>
<ul>
<li>public abstract DLedgerEntry appendAsLeader(DLedgerEntry entry)<br>向主节点追加日志(数据)。</li>
<li>public abstract DLedgerEntry appendAsFollower(DLedgerEntry entry, long leaderTerm, String leaderId)<br>向从节点同步日志。</li>
<li>public abstract DLedgerEntry get(Long index)<br>根据日志下标查找日志。</li>
<li>public abstract long getCommittedIndex()<br>获取已提交的下标。</li>
<li>public abstract long getLedgerEndTerm()<br>获取 Leader 当前最大的投票轮次。</li>
<li>public abstract long getLedgerEndIndex()<br>获取 Leader 下一条日志写入的下标（最新日志的下标）。</li>
<li>public abstract long getLedgerBeginIndex()<br>获取 Leader 第一条消息的下标。</li>
<li>public void updateCommittedIndex(long term, long committedIndex)<br>更新commitedIndex的值，为空实现，由具体的存储子类实现。</li>
<li>protected void updateLedgerEndIndexAndTerm()<br>更新 Leader 维护的 ledgerEndIndex 和 ledgerEndTerm 。</li>
<li>public void flush()<br>刷写，空方法，由具体子类实现。</li>
<li>public long truncate(DLedgerEntry entry, long leaderTerm, String leaderId)<br>删除日志，空方法，由具体子类实现。</li>
<li>public void startup()<br>启动存储管理器，空方法，由具体子类实现。</li>
<li>public void shutdown()<br>关闭存储管理器，空方法，由具体子类实现。</li>
</ul>
<h3 id="1-2-DLedgerMemoryStore"><a href="#1-2-DLedgerMemoryStore" class="headerlink" title="1.2 DLedgerMemoryStore"></a>1.2 DLedgerMemoryStore</h3><p>Dledger 基于内存实现的日志存储。</p>
<h3 id="1-3-DLedgerMmapFileStore"><a href="#1-3-DLedgerMmapFileStore" class="headerlink" title="1.3  DLedgerMmapFileStore"></a>1.3  DLedgerMmapFileStore</h3><p>基于文件内存映射机制的存储实现。其核心属性如下：</p>
<ul>
<li>long ledgerBeginIndex =  -1<br>日志的起始索引，默认为 -1。<br>l- ong ledgerEndIndex = -1<br>下一条日志下标，默认为 -1。</li>
<li>long committedIndex = -1<br>已提交的日志索引。</li>
<li>long ledgerEndTerm<br>当前最大的投票轮次。</li>
<li>DLedgerConfig dLedgerConfig<br>DLedger 的配置信息。</li>
<li>MemberState memberState<br>状态机。</li>
<li>MmapFileList dataFileList<br>日志文件(数据文件)的内存映射Queue。</li>
<li>MmapFileList indexFileList<br>索引文件的内存映射文件集合。（可对标 RocketMQ MappedFIleQueue )。</li>
<li>ThreadLocal&lt; ByteBuffer&gt; localIndexBuffer<br>本地线程变量，用来缓存索引ByteBuffer。</li>
<li>ThreadLocal&lt; ByteBuffer&gt; localEntryBuffer<br>本地线程变量，用来缓存数据索引ByteBuffer。</li>
<li>FlushDataService flushDataService<br>数据文件刷盘线程。</li>
<li>CleanSpaceService cleanSpaceService<br>清除过期日志文件线程。</li>
<li>boolean isDiskFull = false<br>磁盘是否已满。</li>
<li>long lastCheckPointTimeMs<br>上一次检测点（时间戳）。</li>
<li>AtomicBoolean hasLoaded<br>是否已经加载，主要用来避免重复加载(初始化)日志文件。</li>
<li>AtomicBoolean hasRecovered<br> ​    是否已恢复。</li>
</ul>
<span id="more"></span>

<h2 id="2、DLedger-存储-对标-RocketMQ-存储"><a href="#2、DLedger-存储-对标-RocketMQ-存储" class="headerlink" title="2、DLedger 存储 对标 RocketMQ 存储"></a>2、DLedger 存储 对标 RocketMQ 存储</h2><p>存储部分主要包含存储映射文件、消息存储格式、刷盘、文件加载与文件恢复、过期文件删除等，由于这些内容在 RocketMQ 存储部分都已详细介绍，故本文点到为止，其对应的参考映射如下：<br><img src="https://img-blog.csdnimg.cn/20190831215536527.jpg?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70" alt="在这里插入图片描述"><br>在 RocketMQ 中使用 MappedFile 来表示一个物理文件，而在 DLedger 中使用 DefaultMmapFIle 来表示一个物理文件。</p>
<p>在 RocketMQ 中使用 MappedFile 来表示多个物理文件(逻辑上连续)，而在 DLedger 中则使用MmapFileList。</p>
<p>在 RocketMQ 中使用 DefaultMessageStore 来封装存储逻辑，而在 DLedger 中则使用DLedgerMmapFileStore来封装存储逻辑。</p>
<p>在 RocketMQ 中使用 Commitlog$FlushCommitLogService 来实现 commitlog 文件的刷盘，而在 DLedger 中则使用DLedgerMmapFileStore$FlushDataService来实现文件刷盘。</p>
<p>在 RocketMQ 中使用 DefaultMessageStore$CleanCommitlogService 来实现 commitlog 过期文件的删除，而 DLedger 中则使用 DLedgerMmapFileStore$CleanSpaceService来实现。</p>
<p>由于其实现原理相同，上述部分已经在《RocketMQ 技术内幕》第4章中详细剖析，故这里就不重复分析了。</p>
<h2 id="3、DLedger-数据存储格式"><a href="#3、DLedger-数据存储格式" class="headerlink" title="3、DLedger 数据存储格式"></a>3、DLedger 数据存储格式</h2><p><img src="https://img-blog.csdnimg.cn/2019083121572821.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70" alt="在这里插入图片描述"><br>存储格式字段的含义如下：</p>
<ul>
<li>magic<br>魔数，4字节。</li>
<li>size<br>条目总长度，包含 Header(协议头) + 消息体，占4字节。</li>
<li>entryIndex<br>当前条目的 index，占8字节。</li>
<li>entryTerm<br>当前条目所属的 投票轮次，占8字节。</li>
<li>pos<br>该条目的物理偏移量，类似于 commitlog 文件的物理偏移量，占8字节。</li>
<li>channel<br>保留字段，当前版本未使用，占4字节。</li>
<li>chain crc<br>当前版本未使用，占4字节。</li>
<li>body crc<br>body 的 CRC 校验和，用来区分数据是否损坏，占4字节。</li>
<li>body size<br>用来存储 body 的长度，占4个字节。</li>
<li>body<br>具体消息的内容。</li>
</ul>
<p>源码参考点：DLedgerMmapFileStore#recover、DLedgerEntry、DLedgerEntryCoder。</p>
<h2 id="4、DLedger-索引存储格式"><a href="#4、DLedger-索引存储格式" class="headerlink" title="4、DLedger 索引存储格式"></a>4、DLedger 索引存储格式</h2><p><img src="https://img-blog.csdnimg.cn/20190831215844719.png" alt="在这里插入图片描述"><br>即一个索引条目占32个字节。</p>
<h2 id="5、思考"><a href="#5、思考" class="headerlink" title="5、思考"></a>5、思考</h2><p>DLedger 存储相关就介绍到这里，为了与大家增加互动，特提出如下两个思考题，欢迎与作者互动，这些问题将在该系列的后面文章专题探讨。</p>
<p>1、DLedger 如果整合 RocketMQ 中的 commitlog 文件，使之支持多副本？<br>2、从老版本如何升级到新版本，需要考虑哪些因素呢？</p>
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